The present invention relates to a scroll-type fluid machine such as a scroll compressor and, more particularly, to a scroll-type fluid machine in which an outside diameter of the fluid machine is reduced to realize a compact design and a force applied to rotation-prevention mechanism is reduced to provide a higher durability of the machine.
Conventional scroll-type fuid machines include an orbiting scroll member and a stationary scroll member, with the orbiting scroll member having a disk-shaped plate and an involute or substantially involute spiral wrap which protrudes upright from one end surface of the plate, and the stationary scroll member having a construction similar to that of the orbiting scroll member with a discharge port formed in a center of the plate. The orbiting scroll member and the stationary scroll member are assembled together such that the wraps of these members mesh each other while making sliding contact at side surfaces of these wraps. The orbiting scroll member and the stationary scroll member are placed into a hermetic cylindrical casing which is provided with a suction port. The stationary scroll member is fixed to the casing while the orbiting scroll member is driven through a crank pin of a crank shaft connected to, for example, an electric motor while being prevented from rotating about its own axis by a rotation-prevention mechanism, in such a manner that the center of the wrap of the orbiting scroll member revolves about the center of the wrap of the stationary scroll member, i.e., about the center of the involute base circle of the wrap of the stationary scroll member, while maintaining the sliding contact between the side walls of the scroll wraps of both scroll members. As a result of the orbiting motion of the orbiting scroll member, closed spaces formed between the wraps of both scroll members, constituting compression chambers, are progressively moved towards the center of the stationary scroll member while progessively decreasing their volumes, until they are brought into communication with the discharge port formed in the center of the plate of the stationary scroll member. Consequently, gas confined in the compression chamber is compressed to a level higher than the suction pressure before it is discharged through the discharge port in the center of the plate of the stationary scroll member.
Demand is increasing for scroll compressors having smaller sizes in order to meet the requirement for saving installation space and, to cope with such a demand, Japanese Patent Examined Publication No. 56-28239 discloses a scroll compressor which is referred to as scroll compressor of "symmetric eccentricity wrap type". In this type of scroll compressor, the involute angle of the terminal end on the spiral wrap of the orbiting scroll member is the same as the involute angle of the terminal end of the scroll wrap of the stationary scroll member. In addition, the center of the involute base circle of the wrap on the orbiting scroll member is offset by an amount .epsilon./2 from the center of the plate thereof in a direction opposite to the terminal end of the wrap, where .epsilon.=a radius revolution and corresponds to a length of a crank arm of a crank. At the same time, the center of the involute base circle of the wrap on the stationary scroll member is offset by the same amount .epsilon./2 from the center axis of the casing towards the terminal end of the wrap on the stationary scroll member. With such symmetrical offset arrangement of both scroll wraps, it is possible to minimize the diameter of the plate of the orbiting scroll member and, hence, to minimize the inside diameter of the casing. The scroll compressor of symmetrical eccentricity wrap type, which appreciably contributes to the reduction in the size of the casing, suffers from a disadvantage in that the maximum value of the torque applied to the orbiting scroll member, tending to rotate this member about its own axis, is increased due to the radius of the point on which the gas pressure load acts, so that the rotation-prevention mechanism is heavily loaded thereby resulting in increased friction and wear in the rotation-prevention mechanism, thus seriously impairing the durability of the scroll compressor.